Lesson 1/Learning Event 2
a. Loop and Line. The loop extends from the telephone to the hybrid
The line extends between the hybrid units and includes all
facilities within the line circuit.
If the telephones were connected
together directly, the wire connecting them would then be called the line.
The loop is 2W and the line is 4W.
The conversion between 2W and 4W
operation takes place at the hybrid units.
b. Directions of Transmission.
There are two directions of
transmission in a 4W circuit: West to east and east to west (refer to
figure 1-1). The outgoing line is called the send line; it is called the
receive line where it terminates at the distant station. In other words,
the send line at the east station and the receive line at the west station
is the same pair wires.
The normal send loop level is 0 dBm (1mw), and
the normal receive loop level is -3 dB.
Normal net loss in a telephone
system is therefore the difference between these two levels, or 3 dB.
c. Hybrid Loss. The normal signal loss through a hybrid is usually 4
dB in each direction of transmission. The loss results from the splitting
of the energy into two paths, one to the sends line and one to the hybrid
There is a similar loss in the receive path.
A receive line
amplifier compensates for the line and equalizer loss, raising the line
signal level to +1 dBm.
That level drops to -3 dBm on the loop after
passing through the hybrid.